Infectious diseases have re-emerged as a major health problem in North America, in part due to the widespread emergence of antibiotics resistance. The mechanisms of defense against intracellular parasites, and the bacterial strategies underlying survival and replication in host phagocytes remain poorly understood. A better understanding of host defenses against such infections may suggest new strategies for intervention in these diseases. Using a genetic approach, the investigators have identified a new component (Nramp1) of anti-microbial defenses of phagocytes. Mutations at Nramp1 in mice cause susceptibility to several intracellular infections, and polymorphic variants at human NRAMP1 are also associated with susceptibility to Mycobacterial infections in endemic areas of disease. Nramp1 is part of a large family of membrane transporters that has been highly conserved from bacteria to man. Nramp1 is expressed in the lysosomal compartment of macrophages and is targeted to the membrane of bacterial phagosomes soon after phagocytosis. By homology with the known substrates of other Nramp family members, they propose that Nramp1 functions as a divalent cation efflux pump at the phagosomal membrane to suppress bacterial replication. The current proposal has four major goals. The first, is to understand how Nramp1 delivery affects the physiological properties of the phagosome including maturation, acidification, and bactericidal activity of macrophages and neutrophils. The second, is to identify the substrate and mechanism of transport of Nramp1 at the phagosomal membrane. The third is to identify protein determinants responsible for Nramp1 targeting to the lysosome and residues essential for substrate binding and transport. The fourth is to map new mouse loci that affect, in an Nramp1-independent fashion, host resistance to infection with clinically relevant Mycobacteria. Together, these studies should clarify the role and mechanism of action of Nramp1 in phagocytes anti-microbial defenses, which may in turn suggest new avenues for intervention in infectious diseases.